A polarizer is a film able to convert natural light into polarized light having a specific vibration direction, and is employed in various display devices, such as liquid crystal displays, and organic light emitting devices.
The polarizer includes a polarizing film made of a polyvinyl alcohol (PVA) resin dyed with dichroic dye or iodine, and a protective film stacked on one surface or both surfaces of the polarizing film with an adhesive interposed therebetween. Until recent years, a triacetyl cellulose (TAC) film has been widely used as a protective film. However, the TAC film has a drawback in that it may be deformed in a high temperature and high humidity environment. Therefore, in recent years, protective films made of various materials capable of replacing the TAC film have been developed, and for example, the use of polyethylene terephthalate (PET), a cycloolefin polymer (COP), and an acrylic film, alone or mixtures thereof, has been proposed.
Meanwhile, examples of adhesives that may be used for attaching the polarizing film and the protective film to each other include an acrylic adhesive, a dry laminated adhesive prepared by mixing a urethane resin solution and a polyisocyanate resin solution, a styrene/butadiene/rubber adhesive, an epoxy adhesive, a polyvinyl alcohol adhesive, an urethane adhesive, an adhesive containing a compound having a polyester ionomer type urethane resin and a glycidyl group, a thermosetting adhesive, or the like, while water-based adhesives made of an aqueous solution of polyvinyl alcohol resin may chiefly be used.
However, in the case of the water-based adhesives, when an acrylic film or COP film is used as the protective film instead of the TAC film, the adhesive force of the protective film is weak and thus the protective film has a drawback in that use thereof is limited according to the material of the protective film. Also, in the case of the water-based adhesives, in addition to adhesive failure due to the material, when the protective films applied to both surfaces of a PVA polarizing film are made of different materials, curling of the polarizer may occur in the course of a drying process of the water-based adhesives and a lowering of initial optical properties may be caused. As an alternative to solve these drawbacks, a photo-curable non-water-based adhesive has been proposed.
However, since the photo-curable non-water-based adhesives generally have a high viscosity, a final adhesive layer is so thick that when the polarizer is manufactured by using the photo-curable non-water-based adhesives, failure, such as curling in the TD and MD directions may easily occur.
Also, in the case of a double-sided polarizer with protective films attached to both surfaces of a polarizing film, since the adhesive layers formed on both surfaces of the polarizing film should be cured, respectively, light irradiation should be performed twice, which makes the process complicated. While the adhesive layer may be cured by irradiating light thereonto once, the degrees of curing of the adhesive layers are not equal due to a difference between amounts of light arriving at the light irradiation surface and the light non-irradiation surface, and thus the adhesive force of the light non-irradiation surface is low. Thus, the low adhesive force of the adhesive layer deteriorates the durability of the polarizer to thus lower the optical properties of the polarizer.